1. Field of the Invention
The present invention relates to an absolute rotary encoder.
2. Description of the Related Art
An incremental rotary encoder or absolute rotary encoder is conventionally used to measure an angle. The incremental rotary encoder records slits having a predetermined period on a disk, optically or magnetically reads the relative rotational movement of the slits, and computes an absolute position by combining the read result with an origin detecting mechanism. Note that many high-resolution products each of which has a slit pitch decreased to about 80 μm and interpolates phase information within one count by dividing the information into about 10,000 portions by using an electrical divider are recently available. Also, the slits are normally not read one by one, but a plurality of slits are read by optically averaging them. This increases the accuracy because dust, flaws, defects, and pattern errors of the slits themselves are canceled. Since, however, absolute angle information cannot be obtained unless the original is initially detected, applications to particularly the fields of machine tools and robots are limited.
Various kinds of absolute encoders have been proposed as binary patterns, and a so-called gray code method of recording binary patterns on a plurality of tracks and a method of recording a random-number code on one track are available. However, the resolution of the gray code method is not so high because mounting errors make it difficult to synchronize the detection timings of pieces of information of different tracks. Accordingly, a method has been proposed by which binary codes 1 and 0 are expressed by the value of the transmittance, the value of the slit width, or the presence/absence of slits or pits arranged at equal intervals, and recorded as cyclic code patterns on one track. The cyclic code patterns are patterns in which when 1s and 0 s are arranged at random, there are absolutely no portions having the same arrangement on the entire circumference in M adjacent patterns. Japanese Patent Laid-Open No. 60-89713 has disclosed an absolute encoder using an M-sequence code as a kind of the cyclic code.
Japanese Patent Laid-Open No. 2004-529344 has disclosed an absolute encoder using an absolute code obtained by partially removing reflective slits periodically arranged at equal intervals.
When applying the above-described absolute encoder to a rotary encoder, the measurement accuracy decreases if a difference (eccentricity) is produced between the rotational center of a disk and the center of a large number of radially arranged slits. The incremental encoder has conventionally widely used a method by which two detectors are arranged to oppose each other with respect to one disk, and the decrease in accuracy caused by the eccentricity of the disk is canceled by averaging the measurement values of angles. This is so because if the eccentricity amount is much smaller than the pitch of the radially arranged slits, the phases of sine-wave signals from the two detectors can be averaged by a simple method of directly adding the sine waves.
For the absolute rotary encoder, however, no simple method of averaging signals from two detectors has been proposed because the detection of codes and the processing of high-resolution signals are complicated. Furthermore, in a built-in type (module type) rotary encoder, signals deteriorate due to a positional error between a disk and detection head, so it is also necessary to prevent the decrease in accuracy caused by deterioration like this. Under the circumstance, no opposite arrangement type absolute rotary encoder has been implemented.
As described above, earnest demands have arisen for implementing an absolute rotary encoder capable of canceling the influence of the eccentricity of a disk and having both a high accuracy and high resolution. Especially in a so-called module type encoder in which a disk and detection head are separated, the eccentricity of the disk readily occurs. Therefore, an absolute rotary encoder capable of maintaining a high accuracy even if a mounting error such as eccentricity occurs has been desired.